1. Field of the Invention
The present invention relates to a fuel injection control apparatus for internal combustion engines and, more particularly, to a fuel injection control apparatus which injects an appropriate amount of fuel to an internal combustion engine by compensating for response delays of fuel injectors which vary with a battery voltage.
2. Description of Related Art
It is known generally in a fuel injection system for internal combustion engines that fuel injectors for injecting fuel into an engine have a response delay from the time point of receiving a driving voltage for starting fuel injection to the time point of actually opening a valve thereof for fuel injection. They also have another response delay from the time point of shutting off the driving voltage to the time point of actually closing the valve. These response characteristics are shown in FIGS. 7A and 7B. In FIG. 7B, time To represents the response delay of the injector from the time point of receiving the driving voltage of FIG. 7A to the time point of actual valve opening, while time Tc represents the response delay of the injector from the time point of shutting off of the driving voltage of FIG. 7A to the time point of actual valve closing. The response delay time To at the time of valve opening is longer than the response delay time Tc at the time of valve closing.
The injector does not inject fuel during the response delay time To at the time point of valve opening, while it continues to inject fuel during the response delay time Tc at the time point of valve closing. Thus, the actual fuel injection time does not coincide with a required fuel injection time TAU of the injector driving voltage calculated to correspond to a required fuel injection amount. Thus, there occurs a period during which time the injector does not inject fuel during the time TAU of the driving voltage. This time (TV=To-Tc) is referred to as an invalid injection time TV of the injector.
As disclosed in Japanese Patent Publication, Laid-open No. 1-170739, for instance, it is proposed to compensate for the response delays of the fuel injector by adding the invalid injection time TV to the calculated fuel injection time TAU. Because the invalid injection time TV varies with a battery voltage VB as shown in FIG. 8, the invalid injection time TV increases in accordance with a decrease in the battery voltage VB.
Further, the fuel injection system has a pressure regulator which returns excess fuel to a fuel tank when a pressure of fuel PF pressurized and supplied from a fuel pump becomes higher than an intake pressure PM of the engine by a predetermined pressure value. Thus, the pressure difference .DELTA.P between the intake pressure PM and the fuel pressure PF may be maintained constant even when the intake pressure PM varies depending on engine operating conditions. The pressure difference .DELTA.P between the fuel pressure PF and the intake pressure PM is maintained constant at a pressure difference .DELTA.PS, which is equal to the fuel pressure PS set in correspondence with the atmospheric pressure PA, as shown in FIG. 9A. In this instance, since the invalid injection time TV varies only with the battery voltage VB, the injection time may be compensated with high accuracy by determining the invalid injection time TV in accordance with the battery voltage VB and adding the same to the injection time TAU.
This pressure regulator, however, necessitates a return pipe which returns the excess fuel to the fuel tank. The return pipe needs to be extended from an engine compartment, which is normally in the front part of an automotive vehicle, to the rear part of the vehicle where the fuel tank is normally provided, thus complicating the arrangement of the return pipe.
It has been proposed to install the pressure regulator within the fuel tank or in the vicinity thereof to shorten the length of the return pipe and to regulate the fuel pressure PF relative to the pressure in the fuel tank or the atmospheric pressure PA as shown in FIG. 9B.
As understood from FIG. 9B, the pressure difference .DELTA.P between the fuel pressure PF and the intake pressure PM becomes large as the intake pressure PM becomes low towards a vacuum side, although it does not differ much from the set fuel pressure PS as long as the intake pressure PM is around the atmospheric pressure PA. In this instance, the invalid injection time TV will vary not only with the battery voltage VB but also with the pressure difference .DELTA.P. That is, the invalid injection time TV becomes longer as the pressure difference .DELTA.P between the upstream and downstream of the injector becomes larger, because the larger pressure difference exerts a force on the valve of the injector and causes a larger valve opening response delay. Thus, determining the invalid injection time TV in accordance with only the battery voltage VB under the condition that the fuel pressure PF is set in correspondence with the atmospheric pressure and adding the same to the calculated injection time TAU cannot compensate for the response delays correctly. In this case, the air-fuel ratio of air-fuel mixture supplied to the engine deviates to a fuel-lean side, resulting in a decrease in engine output torque and an increase in NOx (nitrogen oxides) in the exhaust gas.